1. Field of the Invention
The present invention relates to a method for sewing together a plurality of separate covering elements that are adapted to undergo a foaming process where a resultant sewn three-dimensional trim cover assembly formed by the covering elements is subjected to foaming with a liquid foaming agent into an integral foamed product.
2. Description of Prior Art
As well known, most of vehicle seats are provided with headrests, armrests or the like. Those equipments for vehicle seat are formed by the steps of providing a plurality of covering elements each preformed in a given shape, sewing them together by a sewing machine into a three-dimensional trim cover assembly, and thereafter injecting a liquid foaming agent into the trim cover assembly, followed by a foaming process where the liquid foaming agent is inflated and cured integrally with the trim cover assembly. In that manner, there is produced a foamed product integral with the trim cover assembly, such as headrest or armrest, wherein the trim cover assembly is integral with a foam padding created therein.
However, during the step of foaming process, it has been frequently the case that a liquid foaming agent is leaked out through needle holes formed by a sewing needle of sewing machine in the trim cover assembly. Hitherto, various attempts have been made to solve such problem. For example, the Japanese Laid-Open Patent Publication No. 8-309765 proposes to effect an overlock sewing along the extreme ends of juxtaposed cover materials, thereby providing an intricately tangled threads around holes formed by a sewing needle along seam with a view to preventing leakage of liquid foaming agent therethrough. The Japanese Laid-Open Patent Publication No. 8-243271 discloses using an elastic filler material along a seam in juxtaposed cover materials. According thereto, the elastic filler material is placed along a given sewing line on the juxtaposed cover materials and a sewing is effected along those juxtaposed filler material and cover materials, so that the elastic filler material is inserted and filled in each of holes formed by a sewing needle along seam in order to prevent leakage of liquid foaming agent therethrough. Further, as disclosed in the U.S. Pat. No. 6,401,643, two different threads are used in a sewing machine for sewing together the cover materials, wherein one of the threads is large in thickness relative to another of them, thereby serving as a leakage preventive member in addition to being a thread.
Nonetheless, the foregoing prior arts have been with the problem that they still require additional separate sewing mechanisms and other separate materials or different threads, thus resulting in troublesome inefficient sewing processes and increased costs of required materials and threads.